This data collection is the result of a Secchi depth data mining study for the North Sea / Baltic Sea region carried out by Thorkild Aarup (t.aarup@unesco.org). It covers the period from 1902 to 1999. The study was completed in 2002.
40,829 measurements of Secchi depth were compiled from the area as a result of this study. 4.3% of the observations were found in the international data centers (ICES Oceanographic Data Centre and the World Ocean Data Center for Oceanography, Silver Spring in the USA), while 95.7% of the data were provided by individuals and ocean research institutions from the surrounding North Sea and Baltic Sea countries. Inquiries made at the World Ocean Data Center for Oceanography Obninsk in Russia suggested that there could be significant additional holdings in that archive but, unfortunately, no data could be made available.
The earliest Secchi depth measurement retrieved in this study dates back to 1902 for the Baltic Sea, while the bulk of the measurements were gathered after 1970. The spatial distribution of Secchi depth measurements in the North Sea is very uneven with surprisingly large sampling gaps in the Western North Sea. Quarterly and annual Secchi depth maps with a 0.5.x 0.5. spatial resolution are provided for the transition area between the North Sea and the Baltic Sea (4.E - 16.E, 53.N - 60.N).